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  • Surgical Pathology Criteria

    Leiomyosarcoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum


    • Malignant neoplasm exhibiting smooth muscle differentiation located in the deep soft tissues, retroperitoneum, mesentery and omentum

    Diagnostic Criteria

    • The definition of malignancy for smooth muscle tumors varies by site
      • The features listed here apply only to the stated sites
    • Smooth muscle differentiation
      • Elongate eosinophilic cytoplasm
      • Pleomorphic or epithelioid cases may require desmin or caldesmon reactivity to support smooth muscle differentiation
    • Any one of the following is sufficient to diagnose a smooth muscle tumor as malignant for the stated sites
      • Cytologic pleomorphism or atypia
      • Tumor cell necrosis
      • In retroperitoneum, mesentery and omentum
        • Mitotic rate >4 or >10 per 50 HPF (results from different published series)
      • In deep soft tissue
        • Mitotic rate >4 per 50 HPF
    • Occasional findings
      • Epithelioid cytoplasm
        • May be clear
      • Multinucleated, osteoclast-like cells
      • Inflammatory infiltrate
      • Palisading / verocay bodies
      • Stroma may range from hyalinized to myxoid
    • Smooth muscle tumor of uncertain malignant potential criteria vary by site
      • In deep soft tissue
        • Bland tumors with 1-4 mitotic figures per 50 HPF
        • Tumors with multiple recurrences
      • In retroperitoneum, mesentery and omentum
        • Bland tumors with 1-10 mitotic figures per 50 HPF
    • Neoplasms with the morophologic features of smooth muscle tumors arising in the bowel wall and expressing CD117 are considered GI stromal tumors

    Richard L Kempson MD
    Robert V Rouse MD
    Department of Pathology
    Stanford University School of Medicine
    Stanford CA 94305-5342

    Original posting:: December 1, 2007

    Supplemental studies


    • Smooth muscle actin and muscle specific actin positive
      • Actin reactivity is not specific
    • Desmin and caldesmon positive
      • More specific than actin but may be less extensive
    • Keratin and EMA may show occasional, usually patchy or dot-like staining
    • S100 has been reported in smooth muscle tumors of various sites
      • The combination of actin negative and S100 positive should strongly suggest nerve sheath tumor
    • CD34 is occasionally positive, usually weakly

    Differential diagnosis


    In the deep soft tissue:
    Leiomyoma (requires all below) Smooth Muscle Tumor of Uncertain Malignant Potential (used for any of below) Leiomyosarcoma (requires any one of below)
    Cytologically bland   Cytologic pleomorphism or atypia
    <1 mitotic figure / 50 HPF Bland but 1-4 mitotic figures / 50 HPF >4 mitotic figures / 50 HPF
    No tumor cell necrosis Multiple recurrences but lacking other atypical features Coagulative tumor cell necrosis

    Malignant Fibrous Histiocytoma Leiomyosarcoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum
    Pleomorphic, histologically undifferentiated Elongate or spindled eosinophilic cytoplasm
    Lacks abundant desmin and caldesmon Typically desmin or caldesmon positive
    MFH may be actin positive
    The distinction between MFH and high grade leiomyosarcoma is not clinically important and may be one of convention

    Dedifferentiated Liposarcoma Leiomyosarcoma of Soft Tissue
    Evidence of concurrent or antecedent atypical lipomatous tumor No evidence of atypical lipomatous tumor
    Foci of smooth muscle cells may be present in dedifferentiated liposarcoma

    Inflammatory Myofibroblastic Tumor Leiomyosarcoma of Soft Tissue
    Usually <14 years, rare >30 years Rare under 12 years
    Chronic inflammation usual Infrequent inflammation
    Rare inflammatory leiomyosarcoma exhibits a regular fascicular pattern

    Gastrointestinal Stromal Tumor (GIST)

    • Tumors arising in the muscularis propria of the GI tract and expressing CD117 should be designated GIST

    Parasitic Leiomyoma

    • Parasitic leiomyoma must be clearly attached to the uterus or other organ
    • In such cases, the criteria for smooth muscle tumors of that organ must be applied

    Mesenteric Fibromatosis Leiomyosarcoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum
    Desmin infrequent Desmin or caldesmon usually positive
    Beta catenin 90% positive Beta catenin negative
    Scant cytoplasm Elongate eosinophilic cytoplasm
    Cytologically bland May be cytologically atypical
    Stroma diffusely myxoid Stroma infrequently myxoid

    Angiomyolipoma/PEComa Leiomyosarcoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum
    May have fatty component Lacks intrinsic fat
    May have abnormal vascular component Lacks distinctive large abnormal vessels
    HMB45, MelanA positive HMB45, MelanA negative


    • Rare
    • Very rare under middle age and in children
    • May be associated with HIV in young patients
    • Most in deep extremities
    • In deep soft tissues, approximately 50% mortality
      • Good prognosis in children if not associated with HIV
    • In retroperitoneum, mesentery and omentum, near uniform mortality
      • Recurs locally and metastasizes


    • According to the guidelines of the ADASP, grading may be prognostically useful
    • French Federation of Cancer Centers System grading scheme for adult sarcomas
      • Tumor differentiation score = 2 for leiomyosarcoma
      • Mitotic index
        • Score 1 0-9 mitoses per 10 hpf (0.1744 sq mm)
        • Score 2 10-19 mitoses per 10 hpf
        • Score 3 >19 mitoses per 10 hpf
      • Tumor cell necrosis
        • Score 0 No necrosis on any slide (one slide per 2 cm tumor diameter)
        • Score 1 <50% of tumor is necrotic on slides examined
        • Score 2 >50% of tumor is necrotic on slides examined
      • Final Grade (add the three scores above)
        • Grade 1 Sum of scores = 2 or 3
        • Grade 2 Sum of scores = 4 or 5
        • Grade 3 Sum of scores = 6 or more

    Use TNM Staging

    The surgical pathology report should contain or address the following:

      • Location
      • Type of resection or biopsy
      • Histologic diagnosis
      • Managerial category
        • III for leiomyosarcoma (Local recurrence common; Metastasis occurs)
        • IIc for smooth muscle tumor of uncertain malignant potential (Local recurrence common, Metastasis can rarely occur)
      • Extent of tumor cell necrosis
      • Grade
      • Stage
      • Size
      • Depth (dermis, subcutis, below fascia, body cavity)
      • Margins
        • Involved
        • Not involved
          • If under 2 cm give all such distances and sites
          • If over 2 cm give minimum distance and site
      • Results of supplementary studies if performed
      • Relationship to other specimens from the same patient


    Neoplasms with predominant smooth muscle differentiation

    Lesions that may demonstrate a prominent hemangiopericytoma-like vascular pattern


    • Kempson RL, Fletcher CDM, Evans HL, Henrickson MR, Sibley RS. Tumors of the Soft Tissues, Atlas of Tumor Pathology, AFIP Third Series, Fascicle 30, 2001
    • Fletcher CDM, Unni KK, Mertens F. Pathology and Genetics of Tumours of Soft Tissue and Bone, World Health Organization Classification of Tumours 2002
    • Weiss SW, Goldblum JR. Enzinger and Weiss's Soft Tissue Tumors, 4th edition, 2001
    • Svarvar C, Bohling T, Berlin O, Gustafson P, Folleras G, Bjerkehagen B, Domanski HA, Sundby Hall K, Tukiainen E, Blomqvist C; the Scandinavian Sarcoma Group Leiomyosarcoma Working Group. Clinical course of nonvisceral soft tissue leiomyosarcoma in 225 patients from the Scandinavian Sarcoma Group. Cancer. 2007 Jan 15;109(2):282-91.
    • Hornick JL, Fletcher CD. Criteria for malignancy in nonvisceral smooth muscle tumors. Ann Diagn Pathol. 2003 Feb;7(1):60-6.
    • Weiss SW. Smooth muscle tumors of soft tissue. Adv Anat Pathol. 2002 Nov;9(6):351-9.
    • Yamamoto T, Minami R, Ohbayashi C, Inaba M. Epithelioid leiomyosarcoma of the external deep soft tissue. Arch Pathol Lab Med. 2002 Apr;126(4):468-70.
    • Farshid G, Pradhan M, Goldblum J, Weiss SW. Leiomyosarcoma of somatic soft tissues: a tumor of vascular origin with multivariate analysis of outcome in 42 cases. Am J Surg Pathol. 2002 Jan;26(1):14-24.
    • de Saint Aubain Somerhausen N, Fletcher CD. Leiomyosarcoma of soft tissue in children: clinicopathologic analysis of 20 cases. Am J Surg Pathol. 1999 Jul;23(7):755-63.
    • Merchant W, Calonje E, Fletcher CD. Inflammatory leiomyosarcoma: a morphological subgroup within the heterogeneous family of so-called inflammatory malignant fibrous histiocytoma. Histopathology. 1995 Dec;27(6):525-32.
    • Billings SD, Folpe AL, Weiss SW. Do leiomyomas of deep soft tissue exist? An analysis of highly differentiated smooth muscle tumors of deep soft tissue supporting two distinct subtypes. Am J Surg Pathol. 2001 Sep;25(9):1134-42.
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